In higher animals, nucleases are secreted by the pancreas and enzymatically hydrolyze nucleic acids to yield, ultimately, the free purine and pyrimidine bases. If not salvaged and re-used, the free bases are degraded further and the end-products excreted. In some vertebrates, including the primates, the dalmatian dog, birds and some reptiles, the end-product of purine degradation is uric acid, whereas in other mammals and reptiles, and also in mollusks, the end-product is allantoin.
The degradation of purines to the end-product, uric acid in man has been intensively studied, since genetic aberrations of this pathway are known. The major purines, adenine and guanine, are first converted into xanthine, which is then oxidized by the complex flavoprotein xanthine oxidase to uric acid and a superoxide radical which undergoes conversion to hydrogen peroxide by the action of superoxide dismutase. In the presence of HPRT, however, a phosphoribosyl group is added to adenine and guanine from PRPP (phosphoribosyl pyrophosphate) to form AMP or GMP with the simultaneous loss of pyrophosphate (PPi), and these may be re-used.
Isotopic studies on vertebrates that excrete uric acid have shown it to derive from both exogenous and endogenous nucleic acids. Only about 0.5 grams of uric acid is excreted daily by the normal person, although up to 5 grams of free purines are formed daily. Evidently, the greater part of the free purines are salvaged or recycled. Uric acid is present in blood largely as monosodium urate, however, both the free acid and the urate salts are relatively insoluble in water, with the result that in some individuals uric acid precipitates and crystallizes in the urine, subsequently forming kidney stones and causing damage to this organ. Uric acid deposits are also formed in cartilaginous tissues, to produce gout, which apparently results from over-production of uric acid. This disease can be alleviated by treatment with the drug allopurinol. Allopurinol is an analog of hypoxanthine. Allopurinol inhibits xanthine oxidase and thus decreases the formation and accumulation of uric acid.
The salvage of purines in mammalian cells is facilitated by the conversion of hypoxanthine (adenine) and guanine to their respective mononucleotide forms, IMP and GMP by HPRT via an ordered, bi-bi reaction mechanism (Davidson, B. L., et al., J. Biol. Chem., 264:520-525 (1989)). A total defect in this gene is the cause of Lesch-Nyhan Syndrome in humans, which is marked by severe retardation, hyperuricemia, hyperuricaciduria, and severe neurological dysfunction (Kelley, W. N., et al., Science, 155:1682-1684 (1967)). Partial loss of this activity results in the over-production of uric acid which subsequently leads to precocious gout and uric acid nephrolithiasis.